Metabolic rate and aging: effects of food restriction and thyroid hormone on minimal oxygen consumption in rats

Metabolic rate (MR) is widely regarded as an important component of aging processes. Decreased MR has been suggested as a possible mechanism of the life-prolonging action of food restriction in rodents and recent reports show lowered plasma levels of tri-iodothyronine (T₃), a regulator of MR, in food restricted rodents. In order to study the relationship between MR, food restriction, thyroid status and aging we measured the Minimal Oxygen Consumption (MOC) of barrier-raised Fischer 344 male rats fed ad libitum (Group A) or fed a diet restricted to 60% of ad libitum intake (Group R). Oxygen consumption (VO₂) was also measured over 24 hr to establish the relationship between MOC and VO₂ under usual living conditions. Results show: (i) MOC declines with age in both groups of rats; (ii) there is no difference in MOC of Groups A and R rats; (iii) MOC of Group R rats is significantly more sensitive to doses of injected T₃ than MOC of Group A rats at all ages; and (iv) MOC of both groups of rats is significantly lower than the lowest VO₂ recorded under usual living conditions. The results indicate that a decrease in MR is not the mechanism by which food restriction retards aging processes in rodents. The results also suggest no change in thyroid status due to restriction of food although there is increased sensitivity to T₃ in food restricted rats.